1. Field of the Invention
The present invention relates to an optical disk drive, and more particularly to a method of detecting a tilt of an optical converging system relative to an optical disk, the converging system applying a laser beam to the optical disk, and to an optical disk drive for correcting the detected tilt.
2. Description of the Related Art
High density recording has long been desired for optical disk drives used with compact disks and computer storages. High density recording methods proposed and adopted heretofore incorporate increasing a numerical aperture of an objective lens which applies a laser beam to an optical disk, using a semiconductor laser capable of emitting a laser beam having a shorter wavelength and hence a finer light spot, increasing a density in the radial direction through land group recording, utilizing multi-layer recording, ultra high resolution techniques, signal processing techniques, and the like.
Of these methods, the increase of a numerical aperture of an objective lens and the short wavelength of a semiconductor laser are direct approaches to high density recording. However, as the numerical aperture of an objective lens increases, the influence of coma increases greatly and the signal read/write characteristics are degraded. The coma is caused by a tilt between the recording surface of an optical disk and the optical axis of an objective lens, more specifically, by a tilt angle .theta. between the optical axis of an objective lens and a normal to the recording surface of an optical disk at the cross point between the optical axis of the objective lens and the recording surface of the optical axis.
The coma Wc to be caused by a tilt between an optical disk and the optical axis of an objective lens is given by the following equation (1) by using the polar coordinates (r, .alpha.) of the cross section perpendicular to the optical axis of a laser beam. EQU Wc=[t(n.sup.2 -1)n.sup.2 sin .theta. cos .theta.]/[(2(n.sup.2 -sin.sup.2 .theta.).sup.5/2 ].times.(NA).sup.3 (r/R).sup.3 cos .alpha.(1)
where t represents a thickness of a disk substrate, n is a refractive index of the disk substrate, .theta. represents a tilt angle between the optical disk and an optical axis (an angle between a normal to the recording surface of the optical disk and the optical axis of an objective lens), NA represents a numerical aperture of the objective lens, and R is an effective radius of the objective lens. The coma therefore increases in proportion with the objective lens numerical aperture raised to the third power and the disk substrate thickness and tilt angle. As the coma increases, the diameter of a spot focussed on an optical disk increases and an error of read/write may occur. It is therefore necessary to detect and correct the disk tilt in order to increase the objective lens numerical aperture NA for high density recording. As a conventional disk tilt detecting and correcting method, techniques disclosed in the publication of JP-A-3-137831 are being made public. According to these techniques, a tilt error detector mounted near at an objective lens detects a tilt of an optical disk, and in accordance with this detected tilt, an objective lens holder supported by a suspension spring is moved and tilted to correct the disk tilt.
With this disk tilt detecting method, however, an optical light source and a photodetector are required in addition to a conventional optical system in order to detect the disk tilt. Furthermore, with this disk tilt correcting method, a mechanism for changing the position of an objective lens is also required. The conventional disk tilt detecting and correcting method is therefore unsatisfactory in that the system becomes complicated and expensive.
In another conventional method disclosed in JP-A-9-83082, corresponding to U.S. patent application Ser. No. 08/697002 filed Aug. 16, 1996 which is hereby expressly incorporated in its entirety herein by reference, a semiconductor laser device is provided with a main electrode and a plurality of subsidiary electrodes, and current injected to the subsidiary electrodes is controlled to correct a tilt between an optical disk and the optical axis of a laser beam. With this method, however, a tilt detecting method and its circuit configuration are not explicitly disclosed.